Networking help

Dan Coutu coutu at snowy-owl.com
Mon Nov 25 16:16:12 EST 2002


pll at lanminds.com wrote:

> In a message dated: Mon, 25 Nov 2002 16:10:41 EST
> "Ken D'Ambrosio" said:
> 
> 
>>I shouldn't be an ARP issue -- if it were, then the other machine sending
>>pings wouldn't work.  Namely:
>>
> 
> Keep in mind.  The pinging machine, systemA *cannot* ping systemC, 
> but *can* ping systemB.  B and C are on the same subnet, A is not.
> 
> By 'cannot ping' I mean, I type 'ping systemC' and it just sits there.
> 
> However, by ssh'ing to systemB, and from there to systemC, I run
> 'tcpdump -i eth1 icmp' and I can see that systemC *is* in fact 
> receiving the "icmp echo request" packets.  systemC just isn't 
> replying to them!
> 


So we need to examine the possibility that System C doesn't know how to 
reach System A even though A does know how to reach C. I'd check netmasks 
on all the systems involved. If I am remembering right you're going from a 
Class A network (10.whatever) to a Class C network (192.168.whatever) here. 
Getting netmasks right is critically important in this kind of environment.

> 
>>I do, however, have to wonder if you're routing correctly.
>>Can you ping the remote subnet's router address?
>>


I very much wonder if it isn't a router configuration problem. I note that 
the 192.168.*.* addresses are reserved for "private" networks that are 
often NAT'd behind a router that does NAT. It can be either really easy to 
setup this or, if setup wrong, really a pain.

I'd first examine very closely the configuration of the router that bridges 
the two networks. If for some reason you find that there are TWO routers, 
or more likely a computer with two NICs bridging the two networks, then 
routing can get really funky as the rogue route sometimes handles traffic 
and sometimes doesn't. I've seen stuff like that cause me sleepless 
nights... :-(
-- 

Dan Coutu
Managing Director
Snowy Owl Internet Consulting, LLC
http://www.snowy-owl.com/
Mobile: 603-759-3885
Fax: 603-673-6676





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